The longtime California senator is in a race with 23 underfunded rivals. One of them will be her single opponent on the November ballot.

Twenty-three largely unknown, underfinanced challengers are vying for the chance to take on U.S. Sen. Dianne Feinstein (D-Calif.) in the fall.

Among the 14 Republicans on the June 5 ballot are Laguna Niguel dentist and lawyer Orly Taitz, 51, who made a name for herself claiming that President Obama was not born in the United States, and Danville autism activist Elizabeth Emken, 49, who has won backing from some GOP leaders and the state party.

The rest of the field, including five additional Democrats and four members of minor parties, are largely unknown to voters. And no one can come close to matching Feinstein's campaign money, Federal Election Commission records show. The senator had raised nearly $9.3 million and had $7.4 million left as of March 31 (updated finance reports will be available Tuesday).

The second-highest fundraiser was Emken, with $301,000 collected and $252,000 remaining.

Two years after high-tech executive Carly Fiorina mounted a vigorous — though unsuccessful — challenge to the state's other U.S. senator, Democrat Barbara Boxer, Feinstein, 78, is expected to have a cakewalk to a fourth full term in November. Political observers say that's partly because of the GOP's low fortunes in California, where Republican registration has dipped to 30% and no one in the party holds statewide office.

But it's also because of Feinstein's full campaign pockets, relative popularity and her longtime presence near the center of the state's political firmament.

"She's pretty popular, the ultimate safe incumbent," said Raphael Sonenshein, who heads the Pat Brown Institute of Public Affairs at Cal State L.A.

Senate challengers tend to prefer to run against the liberal Boxer, who always seems to draw strong reelection challenges from Republicans, Sonenshein said.

"Boxer keeps the Republicans looking for good statewide candidates who think she can be beaten — like Lucy with the football," Sonenshein said.

Bill Carrick, Feinstein's campaign consultant, said "the Republicans just don't have anybody who can run a competitive race unless some self-funder comes to the party" to mount a credible challenge to his client. "Nobody did."

Besides, Carrick said, Republicans nationally are busy fighting for seats in more competitive states than California as the two parties battle for control of Congress and don't want to plowresources into costly uphill races here.

This year's challengers have an additional factor to think about: the state's new primary system, in which every candidate appears on the same ballot, regardless of any party affiliation. Additionally, only the first- and second-place finishers will advance to the November general election.

Feinstein is expected to finish strongly ahead of everybody else but, even if she wins a solid majority, would still face a single opponent in the fall. The candidate who survives the primary and makes even a somewhat decent showing against Feinstein in the second round could help build credibility for a future run for another office.

USC political scientist Sherry Bebitch Jeffe held out the possibility that the new primary system could give an edge to chiropractor Don J. Grundmann, 60, a San Leandro resident running as a member of the American Independent Party.

"We've seen evidence from several past elections where candidates from this party have picked up support because voters got confused and thought they were independent, not Independent," Jeffe said. "With 24 candidates and the open primary, who knows what could happen?"